This invention relates in general to improvements in apparatus and method for measuring the amplitude and phase of sine waves of known frequency. In particular, the invention relates to the measurement of several sine waves which have a coherent frequency relationship and which arrive at a receiver accompanied by noise and/or coherent interference. The first embodiment of this invention is used in conjunction with a receiver for an Omega navigation system.
In the conventional Omega navigation system, three primary navigation frequencies, 10.2 kHz, 111/3 kHz, and 13.6 kHz, are transmitted from eight stations and provide navigation signals blanketing the Earth. A receiver, particularly aboard an aircraft, receives these signals accompanied by noise and interference from atmospheric sources, man-made sources and the interactions between the aircraft itself and the atmosphere through which it flies.
These receivers frequently include a small digital computer used for a variety of tasks. This computer is capable of applying a number of sophisticated processing algorithms to a digital representation of the combined signals and noise to extract the signals from the noise and interference. However, such algorithms can only be effective if a faithful reproduction of the composite signal and noise which appears at the receiver antenna, in the form of an electromagnetic field, is provided to the computer algorithm in digital form.
The present invention obviates these and other problems in the provision of a digital filter utilizing coherent digital summation techniques for the recovery of quadrature components, such as phase and/or amplitude of two or more periodic signals. This digital type filter, which can be used in a wide variety of applications, finds effective employment in a receiver used in an omega radio navigation system. This receiver utilizes the sampling digital filter for processing a transmitted signal comprised of a plurality of pre-established frequencies of interest. In this digital filter, a plurality of band pass filters pass only the frequencies of interest of which are thereafter combined. A sampling and conversion circuit receives the combined signal and generates a digital equivalent thereof at a preselected sample rate. In addition, a digital type processor is operatively connected to the sampling and conversion circuit to determine the phase relationship of the frequencies of interest in the combined signal.
It is therefore the primary object of the present invention to provide a digital filter which is capable of recovering quadrature components, as for example, phase and/or amplitude, of a plurality of periodic signals through digital summation techniques.
It is also an object of the present invention to provide a receiver for omega type radio navigation systems which utilizes digital measuring techniques for the monitoring and comparison of received signals.
It is a further object of the present invention to provide a sampling digital type filter of the type stated and which digital type filter processes a transmitted signal having a plurality of frequencies of interest and at least one subharmonic frequency which is common to the frequencies of interest.
It is an additional object of the present invention to provide a method of determining the location of a mobile receiving station with respect to at least two fixed transmitting stations by converting frequencies of interest in the transmitted signals and processing the digital equivalents of these frequencies of interest.
It is another salient object of the present invention to provide a sampling digital filter of the type stated which is relatively simple in its construction and therefore lends itself to low manufacturing costs and low operational costs.
With the above and other objects in view, our invention resides in the novel features of form, construction, arrangement, and combination of parts presently described and pointed out in the claims.